It is known to use, as a material for producing claddings for optical fibers, a polymer or a copolymer comprising fluoroalkyl methacrylate represented by the formula ##STR1## wherein X is hydrogen atom, fluorine atom or chlorine atom, m is an integer of 1 to 6 and n is an integer of 2 to 10 (e.g. Japanese Examined Patent Publication No.8978/1968)
However, a significantly turbid copolymer is formed by bulk copolymerization of said monomer and methyl methacrylate (MMA). The turbidity of the copolymer is caused presumably because the fluoroalkyl methacrylate having a long molecular chain becomes difficult to diffuse at the latter stage of polymerization.
In preparation of fluorine-containing polymers useful as materials for forming claddings for optical fibers, bulk or mass polymerization is preferably conducted because this method eliminates the need of a suspension stabilizer as used in suspension polymerization and an emulsifying agent as used in emulsion polymerization, and can avoid the possibility of contaminant being introduced during the purification of the polymer.
To inhibit the turbidity, a bulk polymerization method has been proposed for forming a terpolymer (e.g., in Japanese Unexamined Patent Publication No.66706/1986) which comprises:
(1) 30 to 60% by weight of a comonomer represented by the formula ##STR2## (2) 20 to 50% by weight of a comonomer represented by the formula ##STR3## wherein X is hydrogen atom or fluorine atom, and n is an integer of 1 to 4, and PA1 (3) 20 to 50% by weight of MMA. PA1 (a) about 50 to about 95% by weight of fluoroalkyl methacrylate represented by the formula ##STR4## wherein n is an integer of 6 to 10, and (b) about 5 to about 50% by weight of methyl methacrylate; and PA1 (a) about 50 to about 95% by weight of fluoroalkyl methacrylate represented by the formula ##STR5## wherein n is an integer of 6 to 10, (b) about 5 to about 50% by weight of methyl methacrylate, and PA1 (c) 0 to about 40% by weight of at least one of fluoroalkyl methacrylate represented by the formula ##STR6## wherein X and X' each independently represent hydrogen atom, methyl group or ethyl group, Y is hydrogen atom or fluorine atom and m is an integer of 4 to 10, and fluoroalkyl methacrylate represented by the formula ##STR7##
The proposed method, however, is limitative in giving a terpolymer comprising the combination of three comonomers (1) to (3). Further the method gives a turbid terpolymer when using 20% by weight or more of the comonomer (3) and 20% by weight or less of the comonomer (2). Moreover, the method is ineffective in forming a copolymer comprising long-chain fluoroalkyl methacrylate and MMA, or a terpolymer comprising said comonomers and methacrylate having branched fluoroalkyl group. In this case, a turbid polymer is produced by the method.